icx with OpenMP and Undefined Behavior sanitizer

[fvanderhave]

When I combined the Undefined Behavior sanitizer and OpenMP, I got unexpected results from software I am developing (compiling with icx on Linux). I reduced it to the test case in the attached icx_ubsan_omp_issue.c. The OMP for-loop within the OMP parallel section seems to run too many iterations in total when the Undefined Behavior sanitizer is used.

The system I am using has an Intel(R) Core(TM) i5-13600K, so OMP is expected to use 20 threads by default. My system is running Ubuntu 24.04.2 LTS with these compiler versions:

$ /opt/intel/oneapi/compiler/latest/bin/icx --version
Intel(R) oneAPI DPC++/C++ Compiler 2025.0.4 (2025.0.4.20241205)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/intel/oneapi/compiler/2025.0/bin/compiler
Configuration file: /opt/intel/oneapi/compiler/2025.0/bin/compiler/../icx.cfg

$ gcc --version
gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0

$ clang --version
Ubuntu clang version 18.1.3 (1ubuntu1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[fvanderhave]

A run with the expected behavior:

$ /opt/intel/oneapi/compiler/latest/bin/icx -qopenmp -O1 -g -Wall -Wextra -o icx_ubsan_omp_issue icx_ubsan_omp_issue.c
$ ./icx_ubsan_omp_issue
We have entered the parallel section.
Number of threads: 20.
Thread 000 ; inx 0000000 ; &inx 0x7fff4be8643c
Thread 002 ; inx 0100000 ; &inx 0x7155613fbf3c
Thread 004 ; inx 0200000 ; &inx 0x715553ffdfbc
Thread 014 ; inx 0700000 ; &inx 0x71553affa23c
Thread 012 ; inx 0600000 ; &inx 0x7155335fe1bc
Thread 016 ; inx 0800000 ; &inx 0x715539ff62bc
Thread 008 ; inx 0400000 ; &inx 0x715551ff60bc
Thread 006 ; inx 0300000 ; &inx 0x715552ffa03c
Thread 018 ; inx 0900000 ; &inx 0x715538ff233c
Thread 010 ; inx 0500000 ; &inx 0x715550ff213c
Number of elements processed by parallel loop: 1000000.
$

A run with UBsan added, with an unexpected result:
$ /opt/intel/oneapi/compiler/latest/bin/icx -fsanitize=undefined -qopenmp -O1 -g -Wall -Wextra -o icx_ubsan_omp_issue icx_ubsan_omp_issue.c
$ ./icx_ubsan_omp_issue
We have entered the parallel section.
Number of threads: 20.
Thread 016 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 000 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 010 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 005 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 019 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 016 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 003 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 004 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 011 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 002 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 006 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 017 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 013 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 000 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 012 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 010 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 001 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 009 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 005 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 018 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 014 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 015 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 007 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 008 ; inx 0000000 ; &inx 0x7fff0d61257c
Thread 016 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 019 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 005 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 014 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 018 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 017 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 019 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 005 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 016 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 005 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 014 ; inx 0200000 ; &inx 0x7fff0d61257c
Thread 018 ; inx 0200000 ; &inx 0x7fff0d61257c
Thread 017 ; inx 0200000 ; &inx 0x7fff0d61257c
Thread 003 ; inx 0200000 ; &inx 0x7fff0d61257c
Thread 001 ; inx 0200000 ; &inx 0x7fff0d61257c
Thread 011 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 009 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 003 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 005 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 019 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 013 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 008 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 018 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 017 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 000 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 006 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 016 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 001 ; inx 0100000 ; &inx 0x7fff0d61257c
Thread 016 ; inx 0200000 ; &inx 0x7fff0d61257c
Thread 019 ; inx 0200000 ; &inx 0x7fff0d61257c
Thread 006 ; inx 0300000 ; &inx 0x7fff0d61257c
Thread 018 ; inx 0300000 ; &inx 0x7fff0d61257c
Thread 017 ; inx 0300000 ; &inx 0x7fff0d61257c
Thread 000 ; inx 0300000 ; &inx 0x7fff0d61257c
Thread 019 ; inx 0300000 ; &inx 0x7fff0d61257c
Thread 012 ; inx 0300000 ; &inx 0x7fff0d61257c
Thread 011 ; inx 0300000 ; &inx 0x7fff0d61257c

.... many more lines with repeated occurrences of the same value for inx and the same address for inx ...

Number of elements processed by parallel loop: 20000000.

[Mayur_P_Intel]

I am able to reproduce this issue with Intel 2025.1 compiler.

[fvanderhave]

Apologies for not testing with the latest release straight away. The Ubuntu update from 22.04 LTS to 24.04 disabled my oneAPI updates, which I had not noticed. I upgraded my Intel compiler to 2025.1. The unexpected behavior in both the software where I noticed it and in the submitted test program is still present when I test them with UBsan enabled.

[Mayur_P_Intel]

This case has been now escalated as a bug report to the Compiler team.